I sell a good number of 3D printed parts through my Shapeways shop:
Of the various choices for plastics and process at Shapeways, the “Frosted Detail” line is the best for finely detailed parts on our models. But I’ve had issues painting this stuff, such as enamel paints that never cure or flat acrylic paints that cure, but then start showing a stain, as if something oily was wicking into the paint.
And I think that’s exactly what is happening, though it’s been confused with another FD issue, basic cleanliness. A little info about the process will help us to understand the perils:
Frosted Detail is actually made on a 3D Systems ProJet 3500HD printer using their “multi-jet” process. Think of an inkjet print head sweeping back and forth, but spritzing out drops of resin instead of ink. Then add a moving print platform that allows the head to print layer upon layer to build up a 3-D shape. BTW- these droplets are small. Remember when laserjet printers finally removed all the “jaggies” from text and line art? This happened when print resolution finally reached 300 dots per inch (DPI). The 3500HD is capable of printing at over 700 DPI- parts should be near optical quality!
But there’s more to this process. Liquid droplets won’t make a solid part, so the resin needs to be cured. This is done with a flash of UV light on every pass of the print head… and herein lies one of our painting hazards. Think of the resin as a bowl of wet spaghetti noodles, where the noodles correspond to the long polymer chains in the resin. The noodles want to stick together… let them set out a bit, and you’ll have a rock hard bowl of pasta (the strands have “cross-linked”). Add some oil first, and the noodles will stay slippery longer. In our UV-curable resin, we have the equivalent of this oil… a special oil that can be zapped into extinction when it’s hit with UV light. Kill the oil, and the noodles stick.
In multi-jet printing, an entire layer is laid down and zapped all at once. In another process, “stereolithography” (aka “SLA”), a UV laser plays across a pool of the same type of resin, selectively hardening the resin wherever it hits. Now, it’s standard practice in SLA printing to place the freshly printed parts in a UV booth for “post cure”, because the original laser zapping doesn’t fully cure all the resin in the target volume (a few hours in bright sun can do the job too). Hmmm… to my knowledge, Shapeways doesn’t post cure their multi-jet parts, so it seems to me there’s a risk that these parts could contain uncured resin which can leach out over time… and THAT can’t be good for paint!
I was originally blaming another detail of the multi-jet process for my paint woes. These liquid droplets in each layer won’t just hang in space, for example where the part has an overhanging feature. So in each layer, the print head lays down a bit of soft wax wherever the resin isn’t. The next layer’s overhanging resin droplets go on top of this wax, and are supported until they get fused. After printing, this gooey wax (it’s a lot like bacon fat) has to be removed… and Shapeways is not always perfect in doing this job. So it’s up to us to scrub off the last traces of wax, using solvents and/or aqueous cleaners. BTW, you’ll see some surface finish variations on your FD parts. Why? It’s the wax… surfaces “on top” don’t touch the wax, and can appear almost glossy clear. Surfaces in contact with the support wax have a frosty appearance after cleaning. This frost can be scraped or sanded off to improve the finish.
But anyway, I would clean my parts, even using a ultrasonic cleaner, and still my enamel wouldn’t cure. What’s with that? I then found that water based acrylics WOULD cure, so I’ve recommending that ever since. The plastic is an acrylic too, so this seemed to make sense.
Then I made a part that had a printed FD part attached to a brass tube. After painting with a flat grey acrylic, I saw that the paint on the printed part was staining, like it was getting wet, while the paint on the brass tube still looked dry. Again, what’s with THAT?
That’s when I started to suspect non-fully cured resin. For my latest batch of parts, I placed everything under a UV-A lamp (15W fluorescent tube, used for exposing PCB patterns) for an hour. Then everything was cleaned as usual to remove any wax, and painted with spray can enamel primer. It cured, and it’s sticking. Magic!
1. Your Frosted Detail parts from Shapeways may contain uncured resin. Finish the cure with an hour under a UV-A lamp, or a few hours out in the sun.
2. Follow up with a thorough cleaning, using a solvent or aqueous cleaner (like Simple Green).
3. Then paint with the paint of your choice (even lacquer!).